The Breakdown

This encounter use rules closer to a pvp encounter than a pve one, Crowd Control works but it last for a much shorter duration than in normal encounter and is subject to diminishing return. You will need to take on a pvp mindset as well, since the mobs here have no threat table and acts like pvp players. They will assist each other and attack the same target as well as heal the target your are currently working on.

You need to decide your kill order early, and stick to it. As a general rule, killing the healers first is the way to go. You will want to focus all dps on one while the other are locked down.

The shaman if he is present should be the first target. Use heroism early, make sure he has a healing debuff on (Wound Poison, Mortal Strike, etc) and try and control the most aggressive melee fighter (Rogue, Warrior and Death Knight) to diminish their damage. You should probably assign some ranged dps to the pets, since they have low health, they will die fairly easily.

As general rule, the melee npcs are much more dangerous than the caster npcs and as such should have one player assigned to controlling them. Death Knight (Death Grip, Dark Command, Chain of Ice) and Warriors (Intercept, Hamstring, Taunt, various stuns) do a great job at controlling melee npcs. It is important for the entire raid - especially healers - to be aware and ready to move at any time as it is impossible to totally control the melee npc, the best way for them to not damage you is to not be there when they come for you.

Anybody who has a crowd control ability (Mage, Warlock, Druid, Ret Paladin, etc) should be assigned an secondary target that he will try and Crowd Control as much as possible. The goal here is not to lock them down in CC (impossible), but simply to slow them down as much as possible.

Dispels need to be used almost non-stop, the npcs help themselves and will cast many buff and HoTs on each other, you will have to assign several players to dispel duty.

The NPC cannot be tanked, since they drop aggro every second or so, however, they will react to taunt like abilities, switching target and attacking whoever taunted them for about 2 seconds. Once you start killing the healers the fight gets progressively easier and is pretty much won unless you yourself have no healers left.

While it will be nerfed in 3.2.2, at the moment, having Rogues specced into improved Fan of Knives who just spams it non-stop greatly decreases the difficulty of this encounter.

Here's a quick breakdown of how to handle each individual npc class. This is by no mean all inclusive, but simply a guideline and help to get your started:

Class

Tactic

Death Knight Rogue Warrior

Death Knights can easily control those by using Chain of Ice, which has no diminishing cooldown. Taunt also works to a degree as mentioned above. Speccing Unholy for an extra stun from your ghoul pet makes you even more effective.

Warriors can also do a very good job on those, intercept, hamstring and various other stuns make them easily capable of staying on their target. Being prot spec for even more stuns make you even better at this.

Keep in mind that all 3 of those npcs have way to close a gap created between them and their target (Intercept, Shadowstep and Death Grip), you should keep your stuns ready for when they use those, giving the target time to escape. Try and remain vocal on vent to give warning as to who the melee class are targeting, so that they may move away when needed.

Paladin (Melee) Shaman (Melee)

Those are pretty much the same as the Death Knight, Rogue and Warrior above, but are actually easier to control since they lack any 'gap closing' abilities.

Druid (Caster) Mage Priest (Caster) Hunter Warlock

Lower priority target which can simply be assigned a CC to be spammed on them whenever necessary. Having one of your mage set his focus on one of those guys and using polymorph whenever the diminishing return is over is usually plenty. You'll still need to assign one player per target, but their CC doesn't need to be that solid.

If you get the Warlock, you'll have to be aware that his pet can silence and interrupt. It may very well need to die very early or first depending on how difficult it is making thing for your group.

Paladin (Heal) Priest(Heal) Shaman (Heal)

One of those 3 will most likely be your first dps target, and has such doesn't need to be 'crowd controlled'. Any interrupt or stun you can throw at it while killing them helps however.

Those who are not being killed can be locked down quite effectively by a rogue, with their arsenal of stuns and interrupts.

Druid (Healer)

Different from the other healers mainly in the sense that a single Warlock can easily crowd control this guy forever by juggling Fear, Banish, Death Coil and spec specific Stuns.